Polymeric materials are often differentiated into classes by their behavior upon heating: thermoplastics deform and flow at temperatures greater than their melting point, while thermosets remain intractable until the temperature is reached where destructive decomposition occurs. Such a classification scheme works well for polymers formed from highly exergonic reactions that are essentially irreversible; however, polymers that contain readily reversible covalent bonds capable of undergoing rearrangement can be used to create materials that fit neatly into neither category and have beneficial attributes of both. Furthermore, the living nature of such polymerizations causes unique post-polymerization behavior.
Thermoreversible adaptable polymers are materials capable of undergoing a reversible gel-to-sol transition because they incorporate thermoreversible bonds. These thermoreversible covalent bonds are an order of magnitude stronger than hydrogen bonds (Israelachvili, 2002), yet they permit the material to be thermoreversibly transitioned from a crosslinked solid to a non-gelled oligomeric state. As a result, the material is both mechanically strong and readily able to heal fractures and other defects (Chen et al., 2002; Chen et al., 2003). Unfortunately, thermoreversible healing mechanisms are often limited by irreversible side reactions that occur at elevated temperatures (well beyond the sol-to-gel transition temperature). Additionally, strategies for selectively heating a material that is either spatially confined or surrounded by other thermally sensitive materials possess its own set of challenges.
In principle, most polymerizations are reversible. However, realizing depolymerization often leads to complete and irreversible degradation of the polymer. Certain polymers, including those created by radical and ionic polymerization, often depolymerize when heated above a ceiling temperature, which is typically quite high. At such temperatures, irreversible degradation of other molecular structures generally occurs. A few polymers, including poly-(R-methyl styrene) and poly(isobutene), display more moderate ceiling temperatures (61 and 50° C., respectively) (Odian, 1991). In condensation polymerizations, condensate removal favors the forward reaction, thus the retro-reaction is only achieved when the condensate is present in significant quantities.
U.S. Pat. No. 6,933,361, hereby incorporated by reference, describes thermally re-mendable polymeric materials that are made from multivalent furan monomers and multivalent maleimide monomers via the Diels-Alder (DA) reaction. The furan monomers are described as requiring at least three furan moieties and the maleimide monomers are described as requiring at least three maleimide monomers.
A variety of in situ temperature control methods are possible for composite materials, including resistance heating (Park et al., 2008), photothermal particle heating (Sershen et al., 2005), and hysteresis heating. (Ahmed et al., 2006; Suwanwatana et al., 2006). In situ resistance heating employs a resistance element incorporated in the polymer-containing material and linked to an external power supply. Photothermal particles convert visible to near-infrared electromagnetic radiation into heat, and thus the heating is controlled by the combination of light intensity, absorption and wavelength. Although this method is externally triggered, it suffers from the attenuation of light into the material. Hysteresis heating is one of several heating mechanisms that occur when a magnetic material is placed within a magnetic field alternating at radio frequencies (Bozorth, 1978). By selecting appropriate frequencies and particles sizes, these mechanisms may be limited to Neel relaxation. In this process, the magnetic domains attempt to align with the external field while also interacting with neighboring magnetic domains. The interaction between magnetic domains is accompanied by lattice distortion, which has a storage and loss response, the latter producing heat. At temperatures above the Curie temperature, the magnetic domains within a ferrimagnetic or ferromagnetic material randomize, causing the material to undergo a second order phase change to a paramagnetic material. As a result the magnetic susceptibility vanishes and hysteresis heating becomes negligible.
U.S. Pat. No. 5,378,879, hereby incorporated by reference, describes induction heating of a non-magnetic, electrically non-conducting host material through distribution of ferromagnetic or ferromagnetic particles (including iron, nickel and cobalt) within the material and exposure of the host material to an alternating high frequency electromagnetic field. PCT Application No. WO/1991011082, hereby incorporated by reference, also describes methods for providing heat to selected materials by subjecting the combination of the materials and magnetic particles to an alternating magnetic field. Many common ferromagnetic materials, such as Ni, Fe, and Co, have a Curie temperature well above the temperature where irreversible polymer decomposition occurs (i.e., 358, 770, and 1,130° C., respectively) (Bozorth, 1978). While such materials' particles have been successfully used in low concentrations to limit the temperature reached by the bulk polymer, the temperature limitation here is associated with heat transfer rates and becomes a strong function of the sample geometry and shape. Further, regions adjacent to the polymer are likely to reach very high temperatures.
There is a need in the art for novel polymeric materials that may undergo crack-healing or remolding under mild conditions. Such materials would be useful in applications where self-healing of a polymer is needed. The present invention meets this need.